The present invention relates to the field of ultrashort-baseline acoustic navigation systems.
Such systems are used, for example, to locate the positions of underwater pingers, responders, or transponders relative to a surface ship. They typically operate in the 5-50-kHz frequency range and determine the bearing to a target by utilizing the phase differences between the signals received by three or more closely spaced acoustic receiving hydrophone elements. The phase differences indicate the angle between the incoming acoustic wavefronts and the plane of the hydrophone array elements. If the target is a responder or transponder, the target can be located in three dimensions using acoustic travel time to determine the target's range.
The different types of targets that these systems are capable of tracking have different advantages and disadvantages. Pingers are free-running acoustic sources that produce a short burst of fixed frequency acoustic energy (a "ping"), typically at a rate of one to four pings per second. Since pingers are usually battery powered and run continuously, they have relatively short lifetimes (compared with an equivalent-size transponder) and are usually inexpensive, having no receiver or trigger circuits.
Responders are acoustic sources like pingers, but they differ from pingers in that they emit a ping only when they are electrically triggered by, for example, a signal carried by the umbilical cable of a remotely operated vehicle. Responders often receive their power from the vehicle on which they are installed since electrical connections are already required for responder operation, and vehicle power is usually available. Since the time of transmission is known, responders are capable of providing slant-range as well as bearing information.
Transponders are similar to responders in that they are triggered; however, a transponder has an acoustic receiver built into it so that it can reply (with a known delay) to an acoustic interrogation of the correct frequency and pulse length. In this way, a typically battery-powered transponder can operate totally independently of other systems. Like a responder, a transponder can provide the navigation system with slant range as well as bearing.
Compared with pingers, transponders and responders provide a major advantage in the accuracy and flexibility of ultrashort-baseline navigation systems, particularly when the target is not directly underneath the surface ship. This is because the horizontal distance can be calculated much more accurately using known target depth and the measured slant range that can be obtained from pingers and responders than by using hydrophone-derived elevation angle and known target depth, as is required when one uses a pinger. This is true even though the pinger may have a higher update or ping rate.
Furthermore, transponder targets have a much longer operating life because they do not ping continuously; they ping only when interrogated. This is important, for example, when a transponder is left on the sea bottom to mark a location for return at a later date. On the other hand, the maximum update rate for transponders is lower than that for pingers, particularly at longer ranges (up to 2 km), when 2-4-second update periods are common.
As is well known to those skilled in the art, two arrays of hydrophone elements are commonly used in ultrashort-baseline systems. The first array, hereinafter referred to as the "large" array, typically has an element spacing of one to two wavelengths. The wide spacing results in fine bearing resolution but also in an ambiguous fix. For example, with a one-and-a-half-wave-length spacing, the large array produces seven possible fixes.
To resolve the ambiguity, ultrashort-baseline systems also use a second array, hereinafter referred to as the "small" array, whose elements are spaced apart by less than one-half wavelength at the highest operating frequency. Although the narrow spacing does not provide the accuracy of the large array, it provides unambiguous bearing information in the lower hemisphere and so resolves the ambiguity left by the large array.
Conventional systems do not use both arrays simultaneously; ordinarily, common cable conductors have been used to carry the signals from both arrays, and common processing circuitry has been used to process those signals. When such a system begins operation, it responds to the first one or more pings by processing the signals from the small array to obtain a coarse position. It then switches to the large array for subsequent pings to determine position more accurately, occasionally switching briefly back to the small array to insure that it has maintained "lock" on the proper choice among the possible fixes suggested by the large array.
Historically, such systems used pinger targets with update rates of one to four pings per second. This high update rate made it unlikely that the system would lose lock on the correct choice of target location if, for example, the target moved, the ship suddenly changed course, or the acoustic signals were not received for a few pings due to interference. As such systems begin to be used with transponders, however, a limitation in the above approach became apparent. Ultrashort-baseline navigation systems are now being used with transponders whose update rates are on the order of 2-10 seconds, the rate depending on the number of transponders, the interrogation sequence and the slant range.
Whereas their rapid update rates afford conventional systems a reasonable chance of maintaining the correct choice of fix on the large array, newer navigation systems using transponders can lose lock and must revert to the small array. Because some pings must be ignored on account of noise and other factors, it can take several tens of seconds to re-acquire lock.
One way to reduce this problem is to use signals from both arrays simultaneously, i.e., to process the signals from both arrays in parallel so that it is not necessary to switch between arrays. But the straightforward way of employing this approach is economically unattractive. If the system does not switch between arrays, some of the processing circuitry within the hydrophone has to be duplicated, and this increases hydrophone cost. It is important that hydrophone cost be kept as low as possible, however, because the hydrophone is typically located underwater at a position spaced significantly from the ship's hull, so it is subject to being lost or damaged. Moreover, the hydrophone is usually connected by a cable to processing circuitry on the ship, and processing the signals in parallel increases cable cost because it necessitates an increase in the number of cable conductors. Some of the apparatus aboard the ship also must be duplicated.
To avoid such duplication, one might attempt to employ conventional time-division multiplexing. However, the positions in time of zero crossings of the hydrophone signals must be known with great accuracy if the desired bearing accuracy is to be obtained, and this requirement dictates a multiplexer switching rate that would require expensive wide-bandwidth cabling. Moreover, even if the processing circuitry were placed in the hydrophone so that the bandwidth requirement did not increase cable cost, the circuitry for reconstituting the multiplexed signals would itself exert a significant cost penalty.
An object of this invention, therefore, is to obtain the benefits of continuously monitoring both arrays but to do so without suffering the drawbacks of an unnecessary duplication of components or the complication that would result from conventional multiplexing.